Re: New feature "... ALTER CONSTRAINT ... VERIFY USING INDEX"

In Postgres9.1 a new feature was implemented [1] for adding PK and
UNIQUE constraints using indexes created concurrently, but constraints
NOT NULL and CHECK still require full seqscan of a table. New CHECK
constraint allows "NOT VALID" option but VALIDATE CONSTRAINT still
does seqscan (with RowExclusiveLock, but for big and constantly
updatable table it is still awful).

It is possible to find wrong rows in a table without seqscan if there
is an index with a predicate allows to find such rows. There is no
sense what columns it has since it is enough to check whether
index_getnext for it returns NULL (table is OK) or any tuple (table
has wrong rows).